Vinyl chloride resin compositions are widely used as coating materials for electric wires for the wirings in various electric or electronic appliances or in automobiles. Reflecting the improvement in the performance of such appliances and the trend for compact structures in recent years, the coating film is often required to be thin, and consequently vinyl chloride resin compositions are required to have higher heat resistance than before.
When heat resistance is required, it is common to employ a plasticizer having a relatively large molecular weight such as a trimellitic acid ester, a liquid polyester or a dipentaerythritol ester, as the plasticizer for a vinyl chloride resin. However, no adequate heat resistance has been obtained with these plasticizers. For instance, in the case of the trimellitic acid ester which is most popular as a plasticizer for heat resistance, a trialkyl ester wherein the alkyl group has from 9 to 11 carbon atoms (hereinafter sometimes referred to simply as C.sub.9 -C.sub.11), is regarded to provide the highest heat resistance, taking the compatibility with a vinyl chloride resin into accounts. However, even then, there still remains a certain misgiving about the heat resistance when the coating film is thinner than 0.8 mm although it exhibits adequate performance as a plasticizer for a coating composition for so-called 105.degree. C. wires i.e. electric wires having a coating film thickness of about 0.8 mm and a heat resistance at a temperature of 105.degree. C. Namely, it has been regarded as unsuitable for wires required to have heat resistance at a higher temperature, such as 125.degree. C. wires. As a polyester plasticizer, a polyester having a molecular weight of from 2000 to 8000 is practically used. However, this is also limited to a heat resistance level for 105.degree. C. wires. Further, in a certain limited field, a dipentaerythritol ester is practically used as a plasticizer having higher heat resistance than the above-mentioned plasticizers. However, this ester is inferior in the compatibility with a vinyl chloride resin, and it brings about a difficulty in the processability. Thus, it has a drawback that it can not meet the requirements for slender wires or thin coatings. Further, the thermal aging resistance is inadequate.